Visualization is a powerful tool, when done correctly.
Picturing yourself just shooting everyone to the ground is probably not the best approach.
Imagining your move to cover, 360 degree scan,etc. are all helpful.
Coupled with training that emphasizes simplicity of motion and movement, it really will help prepare you for a violent encounter.
Because unless you are truly exceptional...you will not be at your cerebral best during a gunfight. Every "decision" that you have to make will slow you down and take a bite out of your effectiveness.
We talk about gross/fine motor skills, but most time is lost to decisions.
Gross/Fine actions may mean you actually cannot do something under stress...but training will help with that as well.
The importance of training cannot be overemphasized.
Just like owning a piano does not make you a musician...even if you thunk on the keys, you are still just making noise....same with firearms...you just make more noise